PROJECT SUMMARY/ABSTRACT In 2009, strict adherence to the WHO Surgical Safety Checklist, an inexpensive and easy-to-use tool, was found to reduce 30-day mortality from 1.5% to 0.8% and overall complications from 11% to 7% in eight diverse hospitals from Tanzania to the United States. Since then, the Checklist has been adopted by 3,200 hospitals in 93 countries. The benefits of the Checklist, however, depend upon the individual hospitals' ability to implement it effectively. Implementation is the process by which adopting organizations introduce innovations and encourages their use. Implementation may or may not be effective at producing consistent, high-quality utilization of a target innovation. The proposed study will examine implementation processes in a large group of US and international hospitals and will identify factors supportive of effective implementation. The study will also examine the relationship of effective implementation to teamwork, as an explanation for how the Checklist improves outcomes. In order to achieve these aims, data will be collected from 75 surgical units. Data on Checklist implementation processes will be obtained from implementation leaders and surgeons using a survey developed by drawing on prior instruments and preliminary research. Data on implementation effectiveness, teamwork, and Checklist outcomes will be obtained by direct nurse manager, patient safety officer and circulating nurse observations of surgical procedures. Cross-sectional and longitudinal comparisons will relate implementation processes to effective implementation and effective implementation to teamwork and Checklist outcomes. This study will make significant theoretical, methodological, practical and contributions to the field of implementation science. We will contribute to theory about innovation implementation and teamwork by demonstrating that effective Checklist implementation can lead to a virtuous cycle, affecting team dynamics in ways that can continuously improve implementation effectiveness and thus the benefits of the innovation. We will employ novel evaluation tools and training methods that can be applied broadly to increase the likelihood of effective implementation of the Checklist and in future studies of the implementation of similar innovations. For practical purposes, we will identify specific contextual factors and activities that lead to effective Checklist implementation, which can be emulated by other organizations implementing the Checklist and similar innovations.